This invention relates to a system for controlling the pressure of an air supply for a pneumatic direct fuel injected internal combustion engine, and more particularly, to controlling the air supply pressure for such an engine by adjusting the duration of the fuel injection period in each engine cylinder in accordance with the sensed pressure of the air supply.
Air assisted or pneumatic fuel injection systems are currently being used to inject fuel directly into the cylinders of internal combustion engines. With this type of fuel injection, a metered quantity of fuel is deposited in an injector holding chamber in response to a pulsed fuel signal applied to the injector fuel solenoid. At the appropriate time during the engine cycle, a pulsed air signal is applied to the injector air solenoid to open the injector nozzle and start cylinder fuel injection. During the interval of time that the nozzle is open, commonly referred to as the cylinder injection period, pressurized air supplied to the injector drives the fuel from the fuel chamber and forces it directly into the engine cylinder entrained in the pressurized air. The pressurized air serves to atomize the fuel for clean combustion and enables the fuel to be injected directly into a combustion chamber against opposing cylinder compression pressure.
The pressurized air for the pneumatic fuel injection system is typically provided by an air supply having an engine driven air compressor and an air pressure regulator. For reasons of economy and performance, the output of the engine driven air compressor is generally selected to closely match the requirements of the engine with minimal excess capacity. A pressure regulator is generally used to limit the upper pressure of the air supply in a conventional manner by venting excess air to the engine evaporative canister or intake manifold, when the upper pressure limit is exceeded.
At low speeds and light engine loading, it is known that the timing of injector opening (start of cylinder injection) should be as close to cylinder top dead center (TDC) as possible to limit the dispersion of the injected fuel cloud prior to ignition. This maintains the high degree of charge stratification necessary for stable combustion under these engine operating conditions. However, if the injector remains open past the point where the cylinder compression pressure exceeds the regulated air supply pressure, fuel can backflow through the open injector and into the air supply. This contaminating fuel can damage components of the air supply and can also lead to the loss of fuel vapor to the evaporative canister or intake manifold as the regulator vents excess air in regulating the air supply pressure.
In attempting to prevent the fuel from contaminating the air supply system, the conventional approach has been to select a rotational angle before TDC, where the cylinder compression pressure is not expected to exceed the air supply pressure during engine operation, and then set fuel injection timing to ensure that the end of cylinder injection occurs prior to the selected rotational angle. As discussed previously, it is desirable to time the end of cylinder fuel injection as close as reasonably possible to TDC for a given regulated air supply pressure at low engine speeds and light loading. In doing so, it has been found that fuel contamination of the air supply can still occur in certain circumstances. For example, if a leak develops in the air supply, or the pressure regulator becomes dirty or is improperly set, the air supply pressure can drop below cylinder compression pressure at engine rotational angles prior to the end of fuel injection. It has also been found that fuel contamination of the air supply occurs as the air supply pressure drops due to a reduction in air compressor efficiency when the engine operates at higher altitudes.